Fertilizer Facts: April 1997, Number 16

Correcting Physiologic Leaf Spot Damage in
‘Redwin’ and Other Winter Wheat Cultivars

Richard Engel
Assoc. Prof., Soil Fertility/Plant Nutrition
Land Resources and Environmental Sciences Dept.
Montana State University

A winter wheat leaf spot complex of unknown origin that results in tissue necrosis is often observed in the region. Historically, this phenomenon has been referred to as a "physiological leaf spot" by plant pathologists and breeders, meaning it could not be attributed to an infectious pathogen. It has been presumed to result from an unknown metabolic process, or reflect a plant genetic weakness. In Montana, physiological leaf spot has been frequently observed in Redwin hard red winter wheat. Typically, symptoms initially appeared at flag leaf emergence to boot stage. Lesion symptoms are first apparent on the lower or older leaves, and advance to younger leaves after full-emergence. In early stages, lesions are chlorotic (yellow), circular to oblong in shape (1 to 10 mm), with distinct margins. Later, lesions become necrotic (bleached white or gray), their size increases (>10 mm), and they become more numerous. In severely affected plants, spots coalesce and result in premature leaf senescence.

Visual symptoms of physiological leaf spot in Redwin and other winter wheat cultivars are sometimes confused with tanspot and Septoria leaf blotch diseases. However, plant pathogens responsible for these diseases, i.e., Septoria nodorum and Pyrenophora tritici f. sp. repentis, can not be isolated from affected tissue. Also, field studies have revealed foliar fungicides, e.g., propiconazole (Tilt), have no effect on leaf spot severity (lesion size or numbers) (Table 1). Winter wheat cultivars differ greatly in their susceptibility to physiologic leaf spot. In side-by-side comparisons, specific winter wheat varieties will exhibit leaf spot symptoms while other varieties, grown at the same location, do not. ‘CDC Kestrel’ is the most leaf spot susceptible winter wheat cultivar, followed by ‘Promontory’, Redwin, and ‘Sierra’. ‘Stephens’ and ‘Manning’ are also affected, but their susceptibility is not as great as Redwin.

Table 1. Applicaton of propiconazole (Tilt) on physiologic leaf spot severity in Redwin and CDC Kestrel winter wheat at three locations south of Lodgegrass.

Field research has revealed that control of physiologic leaf spot in Redwin and other susceptible cultivars is consistently achieved through application of chloride (Cl) containing fertilizers. Visual responses to Cl are particularly impressive in cultivars with considerable susceptibility (e.g., CDC Kestrel), or at sites where there has been considerable leaf spot damage. A summation of results, from ten locations where Redwin winter wheat was grown, illustrates the relationship between soil Cl and leaf spot damage (Figure 1). At sites with soil Cl levels >10 lbs/a (0-24 in. depth), damage from physiologic leaf spot is minor. As soil Cl drops below a 10 lbs/a threshold level, leaf spot

severity increases exponentially. Similar relationships are observed in other leaf spot susceptible cultivars, except the threshold levels for leaf spot damage differ slightly. For the most susceptible cultivar, CDC Kestrel, the threshold soil Cl level (0-24 in. depth) is approximately 20 lbs/a.

Chloride fertilization has consistently increased winter wheat grain yield and mature kernel weights at sites with physiological leaf spot (Table 2). Yield responses to applied Cl averaged 6.9 bu/a or 10%, but can vary considerably with location. Thousand kernel weight increases from applied Cl averaged 7.2%, hence kernel size is the most important yield component affected by Cl. Leaf spot suppression by Cl may explain a portion of the yield responses. A reduction in photosynthetic area due to lesion formation, especially in the flag leaf, could affect yield by reducing kernel size at harvest.

Physiologic leaf spot is a problem of low Cl soils. To prevent potential damage in susceptible varieties, fertilize to increase soil analysis (not all soil labs perform soil Cl tests) it is recommended that growers

apply 20-25 lbs/a Cl (40-50 lbs/a 0-0-60 material) to fields where physiologic leaf spot has been a problem. Granular 0-0-60 is the most economic source of Cl to apply ($0.17/lb Cl), and the material application cost of this recommendation is about $4/a. This material can be blended with nitrogen, and broadcast applied in the fall or early spring to produce satisfactory results. While yield responses to Cl fertilizer at sites with physiologic leaf spot are variable, the cost of correcting physiologic leaf spot through 0-0-60 fertilization is small when contrasted with the potential for improved yields and economic return.

Fertilizer facts

• Applying chloride containing fertilizers (e.g., 0-0-60) prevents leaf spotting in Redwin and other winter wheat varieties.
• Application rates of 20-25 lbs/a Cl, or 40-50 lbs/a of 0-0-60 material, are recommended.
• Yield and economic return is improved by correcting potential leaf spot problems with chloride fertilization.

Edited by Jeff Jacobsen, Extension Soil Scientist

Table 2. Chloride fertilizer effect on grain yield and mature kernel weight in leaf spot susceptible winter wheat varieties.

†Site 1 = Garryowen, 1991; 2= Lodgegrass, 1991; 3-6 = South of Lodgegrass, 1992-1995, respectively; 7=Fort Smith, 1995.
‡40 lbs Cl/a at Sites 1-3; 60 lbs Cl/a at Sites 4-7.